Patent GB-601877-A (GB)
Title: Electric bridge circuits for measuring resistances
IPC: G01R17/10
Filed: 1945-10-09
Abstract: 601,877. Resistance measurements ; thermionic valve voltmeters. WELWYN ELECTRICAL LABORATORIES, Ltd., and DENES, P. Oct. 9, 1945, No. 26361. [Class 37] Apparatus for measuring resistances comprises an A.C. Wheatstone bridge, an A.C. amplifier for the output from the bridge, and an indicator actuated by the amplifier, the amplifier containing a thermionic valve with means for maintaining a negative bias on the control grid so selected that the optimum amplification is obtained at the crucial point in the variation of resistance. A.C. voltage from a source 16, Fig. 2, is applied across the diagonal 12, 14 of the bridge, the output from the diagonal 13, 15 being applied to an A.C. amplifier having three stages of amplification. The anode of the first amplifying valve 19 is connected to the positive H.T. main 20, the cathode being connected to the H.T. negative main 21. The anode output is connected through a blocking condenser 22 to the control grid of the second valve 23. The control grid and cathode of this valve are also connected to the negative end 24 and a tapping contact 25 of a potentiometer 25 connected across the H.T. main 20, 21. A third valve 27 gives a third stage of amplification, its output being connected to the indicator 18. Assuming that the voltage from the first valve 19 due to unbalance of the bridge is 25 volts and that an increase of 0.1 per cent of the measured resistance from a nominal value increases the output of the first stage to 25.5 volts, if a negative bias of - 26 volts is applied to the grid of the valve 23 amplified bridge outputs of up to 25 volts will not appreciably affect subsequent stages but change in bridge output corresponding to the extra 0.1 per cent unbalance will be fully amplified in the following stages. By choosing a suitable negative bias bridge outputs below a predetermined limit of bridge unbalance will not be appreciably amplified by the later stages of amplification, but further bridge unbalance will be fully amplified and affect the final indicator. As shown in Fig. 4 the output from the amplifier is passed through a blocking condenser 30 and resistance. 31 to the control grid of a gas-filled relay 32 normally biassed well beyond the cut-off point so that it is not energized when H.T. is applied through the operator's switch 33. When the amplifier output is above a certain amplitude the positive peaks swing the grid voltage of the gas-filled relay 32 below the critical voltage and the relay is energized when the H.T. is applied, so that if the resistance under test does not lie within the precribed limits a relay 37 is energized and operates a lamp or other indicator. In a modification, Fig. 5, an indication is given if the resistance under test falls within the prescribed limits. The bridge output is passed through a rectifier 34 and resistance 36 is applied to the grid of a gas-filled relay 32 so biassed that the relay 37 is energized in the absence of auxiliary bias from the amplifier. The amplifier output causes a voltage to appear across the resistance 36 and increase the negative bias. If the amplifier output exceeds a predetermined limit the gas-filled relay 32 is biassed beyond cut-off and the relay 37 is not energized. In that case the indicator 18 itself is energized when the unbalance of the bridge represents a resistance which is within the desired accuracy.